1. Field of the Invention
The present invention relates to a piston for an internal combustion engine.
2. Description of the Prior Art
A piston for an internal combustion engine has been proposed in Japanese Unexamined Patent Publication (KOKAI) No. 162014/1979. The piston includes a resin coating which is provided on an entire sliding surface of a skirt portion thereof.
Further, another piston for an internal combustion engine has been proposed in Japanese Unexamined Utility Model Publication (KOKAI) No. 65360/1985. The piston includes slits which are formed in a resin coated surface. The resin coated surface covers an entire sliding surface of a skirt portion of the piston, and the slits are adapted to be non resin coated portions in the resin coated surface.
The first conventional piston which includes the resin coating provided on the entire sliding surface of the skirt portion is effective in the reduction of the sliding resistance. However, since the resin is far inferior to aluminum, i.e., the mother material of the first conventional piston, in hardness and the thermal resistance, the resin coating loses the function of the sliding resistance reduction when it is worn out. The resin coating is worn out because of the following reason. Namely, since the entire sliding surface of the skirt portion is coated with the resin coating, the piston comes to be operated without a lubricating oil. In other words, when the piston is operated, for instance, under a high temperature and high load condition, the mother material expands thermally so that there is no clearance between the piston and a cylinder bore. Accordingly, an oil film is more likely to break up. Thus, the resin coating is worn out rapidly when it is subjected to the above operating condition.
The second conventional piston includes the slits which are non resin coated portions in the resin coated surface. It is believed that the slits are formed by machining, etching, or the like. However, such a process for forming the slits requires a very time-consuming operation, and accordingly it results in an increased production cost.